1. Field of the Invention
This invention relates to a magnetic transfer ring method for transferring information signals recorded on a master medium from the master medium to a slave medium.
2. Prior Art
As a method for duplicating a magnetic recording medium, on which video or audio signals are recorded previously, there is known a so-called magnetic transferring method, according to which a slave medium is placed in contact with a pre-recorded master medium, with the magnetic layers thereof in intimate contact with each other, and a bias magnetic field is applied under an optimum contacting state for transferring the magnetic recording on the master medium onto the slave medium.
Heretofore, when transferring magnetic recording by a magnetic transferring method, an apparatus adapter for impressing a bias magnetic field in the longitudinal direction of the medium, as described for example in the Japanese Patent Kokoku Publication No. 52-36004(1977), was used. Recently, there has come to be known an apparatus making use of a transfer drum formed of soft magnetic iron, as shown for example in Japanese Utility Model KOKAI Publication No. 63-44214(1988). This apparatus has an advantage that, since the bias magnetic field is impressed with emphasis in the perpendicular direction, it becomes possible to suppress demagnetization of the master medium by the bias magnetic field. Thus the apparatus is practically employed for preparing a so-called soft tape such as a video tape.
Meanwhile, with the above described magnetic transferring method, which is usually employed for mass production of soft tapes, it is required that the master medium undergoes lesser demagnetization even after repeated transfer operations. Demagnetization of the master medium depends on the bias magnetic field which is applied during the transfer operation. Thus, for suppressing demagnetization of the master medium, it is desired that the strength of the bias magnetic field necessary for the transfer operation be as low as practicable.
On the other hand, coercivity Hc.sub.2 of the slave medium is proportional to the strength of the bias magnetic field to be applied for the transfer operation. Thus, for suppressing demagnetization of the master medium and simultaneously increasing the transfer efficiency, it was felt that coercivity Hc.sub.2 of the slave medium and coercivity Hc.sub.1 of the master medium be related to each other by the formula (1) ##EQU1##
However, considering the anti-magnetic field in the perpendicular direction of the medium, coercivity in the longitudinal direction of the medium (longitudinal coercivity) is not critical. More specifically, in a medium having a longitudinal rectangular ratio R.sub.s closer to unity and lower switching field distribution (SFD), such as, for example, a Co-containing .gamma.-Fe.sub.2 O.sub.3 tape or metal tape, it may occur that, while the longitudinal coercivity of the medium and the strength of the longitudinal reverse magnetizing field thereof coincide fairly well with each other, the perpendicular coercivity of the medium and the strength of perpendicular reverse magnetizing field thereof differ significantly from each other. That is, with the magnetic transferring method, when the bias magnetic field is impressed in the longitudinal direction, if the relation ##EQU2## is satisfied, it becomes possible to reduce demagnetization of the master medium by the bias magnetic field to thereby obtain satisfactory characteristics. However, through our repeated experiments, it has now been found that, when the bias magnetic field is impressed in the perpendicular direction, demagnetization of the master medium can not necessarily be suppressed even when a master medium-slave medium combination satisfy the relation ##EQU3## or the relation ##EQU4## is used and an optimum bias magnetic field for the slave medium is impressed for the transfer operation. Thus, with the magnetic transferring method in which the bias magnetic field is impressed in the perpendicular direction, a new information concerning the condition to be satisfied by the master medium and the slave medium is necessary for suppressing demagnetization of the master medium and improving the transfer efficiency.